Method of producing quartz glass bodies

ABSTRACT

In a known process for the production of quartz glass bodies, SiO 2  particles are deposited of the mantle surface of a cylindrical carrier rotating about its longitudinal axis, forming an elongated, porous preform, where the SiO 2  particles are formed in a plurality of flame hydrolysis burners which are arranged in at least one burner row parallel to the longitudinal axis of the carrier and are moved at a preset translational speed forward and back between turnaround points at which points their direction of movement is reversed, and in which process the preform is sintered. In order to make available on this basis an easily accomplished process that makes it possible to manufacture a preform which is largely free of localized density variations, the invention proposes on the one hand that the base value of the surface temperature of the preform being formed be kept in a range between 1,050° C. and 1,350° C., that the average peripheral velocity of the preform be kept in the range between 8 m/min and 15 m/min and the average translational velocity of the burner row be kept in a range between 300 mm/min and 800 mm/min. On the other hand, the object is also accomplished according to the invention and on the basis of the known process in that in the area of the turnaround points (A, B) the peripheral velocity of the preform being formed is increased and/or the flame temperature is lowered and/or the distance of the burners from the preform surface is changed.

FIELD OF THE INVENTION

The invention relates to a process for the production of quartz glassbodies by deposition of SiO₂ particles on the mantle surface of acylindrical carrier rotating about its longitudinal axis and forming anelongated porous preform, where the SiO₂ particles are formed in aplurality of hydrolysis burners which are arranged in at least oneburner row disposed parallel to the longitudinal axis of the carrier andbetween turnaround points at which points the direction of burnermovement is reversed, where the burners move at a preset translationalspeed forward and back, and by the sintering of the preform.

DISCUSSION OF PRIOR ART

Such a process is described in EP A1 0 476 218. In the known process,SiO₂ particles are deposited in layers by flame hydrolysis burners on ahorizontally oriented substrate rod which rotates about its longitudinalaxis. The burners are installed equidistantly 10 cm apart on a burnerblock extending parallel to the longitudinal axis of the substrate rod.The burner block is moved back and forth along the porous cylindricalpreform which is being formed during the SiO₂ particle deposition, andbetween a left and a right turnaround point. The amplitude of thetranslational motion is lesser than the length of the preform. Due tothe slowing down of the translational motion of the burner block at thepoints of directional reversal an overheating of the preform surfacetakes place and with it a localized axial density variation develops.This creates areas of differing reactivity of the preform which makethemselves known especially during subsequent chemical reactions infurther treatment of the preform and which can result innon-homogeneousness of the quartz glass body after the sintering of thepreform.

EP A1 0 476 218 proposes to solve this problem by continuouslyrelocating the turnaround points of the burner block motion in relationto the preform and thus distributing them evenly over the preform. Forthis purpose both the left and the right turnaround points are relocatedby a few millimeters at every burner pass.

However, this merely results in an even distribution in the preform ofthe localized density variations which develop at the turnaround points.In addition, the complicated translational motion of the burner blocksin this known process requires a high expense related to the apparatusand the controls.

SUMMARY OF THE INVENTION

Therefore the object of the present invention is to make available aneasily implemented process which would make possible the production of apreform largely free of localized axial density fluctuations.

Based on the process described above, the object is accomplishedaccording to the invention in that the base value of the surfacetemperature of the preform being formed is kept in a range between1,050° C. and 1,350° C., that the average peripheral velocity of thepreform is kept in the range between 8 m/min and 15 m/min and that theaverage translational velocity of the burner row is kept in a rangebetween 300 mm/min and 800 mm/min.

The surface temperature of the preform is measured at the point of flameimpingement of one of the central hydrolysis burners in the burner row.A "Infratherm IN 4/5" pyrometer made by IMPAC and having a measurementwavelength of 5.14 μm is used for this purpose. The measurement area isapproximately 5 mm at a distance of the pyrometer from the preformsurface of 30 cm at a temperature between 500° C. and 1,300° C. Theburner flame impingement point has a somewhat greater diameter of about15 m. When the pyrometer is properly adjusted the measurement point iswithin the flame impingement area. A mis-adjustment results in ameasured value that is lower than the actual temperature. The measuredtemperature resulting from correct adjustment will be used as thesurface temperature hereinafter.

This temperature value substantially determines the density of theporous preform. A deposition of SiO₂ particles at the indicatedtemperature range normally results in average relative densities of thepreform between 15% and 35% in relation to the density of quartz glass(2.2 g/cm³). Ten equidistant test drill cores with a diameter of 3 cm,arranged along the length of the preform are taken to establish theaverage relative density of the preform as exactly as possible. Thecores are measured by means of a mercury pycnometer. The averagerelative density of the preform is then derived from the arithmeticaverage value of these measurements, in relation to the above indicatedtheoretical density of quartz glass.

The base value of the surface temperature is substantially determined bythe flame temperature of the hydrolysis burners, their quantity anddistance from the preform surface, as well as their size.

That value can be easily set by an expert in a range between 1,050° C.ad 1350° C. Several rows of burners can also be used for the formationof the porous preform. The points of burner flame impingement of thevarious rows can extend along a common line on the preform surface. Theburners of different rows can be arranged at one height, or offset, asseen in vertical direction relative to the cylinder axis of the carrier.In the case of multiple rows it is sufficient to determine the surfacetemperature base value in one of the rows.

It has been shown that at a base surface temperature value between 1,050and 1350° C., a relatively low increase of the surface temperature ofmax. 150° C. in relation to the base value is attained at the turnaroundpoints when the above indicated velocity range of the preform rotationor the average translational velocity of the burner row is maintained. Atemperature increase of about 150° C. corresponds to a maximal increaseof the relative density of about 6%. Such axial density gradient in thepreform does not reduce the usefulness for many applications of quartzglass bodies subsequently produced from the preform after the sintering.Naturally, the smaller the temperature difference is kept between thebase value and the maximal value at the turnaround points, the lower theincrease of the relative density.

It has been shown to be significant that the motion of the burnersrelative to the preform is on average relatively small (when viewed overthe entire motion cycle). Based on this realization and the averagetranslational velocity ranges of the preform rotational speed and theaverage translational speed of the burner row, an expert can optimizethese steps by means of a few experiments to such an extent that thetemperature increase does not exceed 150° C. over the base value of thesurface temperature. Due to the fact that one or both of the velocitiesin question are set based on a relatively small average value during theforward and back motion (hereinafter "motion cycle"), the preformsurface in the vicinity of the flame impingement point is well heatedthrough. The difference between the well warmed-through surface and thehigher temperature in the turnaround region is therefore relativelysmall. The relative temperature increase in the turnaround region (ascompared to the well warmed-through surface of the preform) is thereforethe smaller the lower the velocity of the relative motion between thepreform surface and the burner row in the middle.

The average translational velocity of the burner row is defined as theratio of the distance covered during one motion cycle and the timerequired for it.

The average peripheral velocity of the preform being produced is derivedfrom the distance covered by every point on the surface of the preformduring the motion cycle relative to the time required to cover thatdistance.

Typically the average translational velocity is kept constant during theentire deposition process. It should be noted that at the turnaroundpoints the translational velocity equals zero and in practice theslowing down of the burner row requires a stopping distance and theacceleration requires an acceleration distance during which the averagetranslational velocity necessarily cannot be in effect.

The average peripheral velocity can also be held constant during theentire deposition process. In this case the rotation velocity of thecarrier must be continually decreased since the outer diameter of thepreform and with it the cylinder mantle surface continually increaseduring the deposition process.

Both the translational velocity and the peripheral velocity can bevaried during the motion cycle, for example the peripheral velocity inthe area of the turnaround points can be increased.

In kinematic reversal, instead of the burner row, or of course inaddition to it, the preform can also be moved back and forth, where theninstead of the translational velocity of the burner row it is thetranslational velocity of the preform or the relative speed between theburner row and the preform surface that needs to be observed.

It has been shown to be disadvantageous when the preform area betweenadjoining turnaround points can cool to too low a temperature during amotion cycle. Such disadvantageous cooling is prevented by the aboveindicated minimum velocities of the outer preform and/or the averageburner row translational velocity.

Axial density gradients in the preform are largely avoided by theprocess according to the invention. Therefore the issue is beyond amerely even distribution of density gradients in the preform asdescribed in prior art. A relocation of the turnaround points is notnecessary for this so that equipment and control device expenditures canbe kept low. However, a relocation of the turnaround points is possibleas a supplementary technique.

It has been shown to be advantageous to maintain the distance ofadjoining turnaround points between 5 cm and 40 cm. This helps toprevent an excessive cooling of the preform surface between adjoiningturnaround points during a motion cycle.

It has also been shown to be advantageous in this regard to use awarming burner between every two adjoining burners in a burner row. Thewarming burners are moved according to the motion of burner row andshorten the distance between heated areas on the preform. They can bearranged centrally between the hydrolysis burners. For this purpose thewarming burners can be arranged on the same burner row or on a separatewarming burner block being moved synchronously to the burner row whichmoves along the carrier.

Further, the above-mentioned object on the basis of the process of thekind described is attained according to the invention in that in theturnaround point zone the peripheral velocity of the developing preformis increased and/or the flame temperature of the hydrolysis burners islowered and/or the distance of the hydrolysis burners from the preformsurface is changed.

By means of each one of these measures, or by means of a combination ofthe measures, a temperature increase of the preform surface in theturnaround point zone can be fully or partially compensated. Thetemperature increase in the turnaround zone thus does not exceed thebase value defined above, or does so only slightly. This has the resultthat both in terms of time and space the preform is heated as evenly aspossible over its entire length. In this way axial density gradients inthe preform are largely avoided.

Change of the listed parameters in the turnaround zone can take place ina regulated or controlled manner. The magnitude of the required changedepends on numerous given factors, for example on the actual parametervalues, the base value of the preform surface temperature, or on theacceptable axial density gradient of the preform. However, changes ofthe parameters in a concrete example can be easily optimized by anexpert following a few experiments based on the teaching provided here.

Increase of the peripheral velocity refers to the average translationalvelocity as defined above. Due to the increase of the peripheralvelocity each burner in the turnaround zone covers a larger area of theforming preform per time unit. This reduces the heating output persurface unit and thus the temperature increase in the turnaround zone.

Reduction of the burner flame temperature refers to the flametemperature set in the center of the area between the turnaround points.

Distance of the burners from the preform surface can be increased orreduced. A reduction or increase of the distance can result in loweringof the surface temperature if the burner flame at impingement pointbecomes colder as a result. This can be the case particularly withso-called focusing burners. The distance is measured between the burnerorifice and the preform surface.

The zone around the turnaround points where these additional measuresare useful starts at a few millimeters around each turnaround point.However, the zone can also extend beyond the center of adjoiningturnaround points as will be explained below in more detail by way of anexample where the parameter in the transitional zone is being changedcontinually.

An axial variation of the burner row turnaround points can also bedispensed with in this process. Therefore the apparatus-relatedexpenditures in comparison to the process known from prior art are lowdespite the required regulating or control devices. However, a variationof the burner turnaround points is possible as a supplementary measure.

In a preferred implementation of the method a reduction of the flametemperature is achieved by reducing the rate of fuel gas supply to theburners in relation to the other gases supplied to the burners. Fuelgases are those gases whose exothermal reaction with one anothersubstantially feeds the burner flame. In an oxyhydrogen gas burner thesefuel gases are for example oxygen and hydrogen, which will subsequentlybe assumed for the sake of simplicity. A reduction of the flametemperature is achieved by either reducing the rate of oxygen and/orhydrogen supply to the burners or by supplying, or increasing the rateof, supply of other gases, such as for example of inert gases or of thestarting material for the formation of the SiO₂ particles.

It has been shown to be particularly valuable in this context to makethe following changes gradually: increase of the peripheral velocity ofthe growing preform, reduction of the burner flame temperature and/orchange of the burner distance from the preform surface at thetransitional areas ending within, before, or at the turnaround points.The gradual change leads to homogenous change between the preform areasin the turnaround point zones and the other areas of the preform. Theparameter requiring change is adjusted to the desired value within thetransitional areas. The adjustment of the value can terminate exactly atthe turnaround point or before. The transitional zones each extend toboth sides of the turnaround points. When the burner row moves away fromthe turnaround points the previously modified parameters are graduallyreturned to their original values. The transitional areas generallycommence at least 10 mm before each turnaround point since in the caseof smaller transitional areas the effect of a gradual parameter changeis hardly noticeable.

When moving away from the turnaround points, the burner rows traversepreform areas which still exhibit a high surface temperature as a resultof the opposite motion. It has been shown to be therefore advantageousto set a shorter transitional area for the burner row motion toward theturnaround points than for the reverse motion. Due to the longertransitional area each parameter is returned to the original value at aslower rate. In this manner an excessive heating of the still-hotpreform surface is avoided as much as possible.

Advantageously, the preform surface temperature is measured in the flameimpingement area of a burner and the measured value is used to establishthe rotational velocity of the carrier, the flame temperature of theburners and/or the distance of the burners from the preform surface.This measure helps to avoid an excessive temperature increase at theturnaround points and makes maintenance of a constant preform densityover the entire duration of the deposition possible.

An alternative process has also been shown to be advantageous, where therotational velocity of the carrier, the flame temperature of the burnersand/or their distance from the preform surface is controlled. Thecontrol is adjusted such that during every motion cycle one or moreparameters in the turnaround point area are changed equally. A controlis especially preferred when regulation is not useful or is onlypossible at high regulating technology expenditure.

A combined approach has been shown to be particularly advantageous,wherein on one hand the base value of the surface temperature of thegrowing preform is kept between 1,050 and 1,350° C., the averageperipheral velocity of the preform is kept between 8 m/min and 15 m/minand the average translational velocity of the burner row is kept between300 mm/min and 800 mm/min, and where additionally, on the other hand, inthe turnaround point area the peripheral velocity of the growing form isincreased and/or the flame temperature of the burners is lowered and/ortheir distance from the preform surface is changed. The advantageousembodiments of the individual approaches according to the inventiondescribed above in more detail have also been shown valuable for suchcombined approach.

Both the maintenance of the surface temperature and the above listedvelocities, as well as the change of the above-named parameters in theturnaround point areas, are geared toward a warming of the preform thatis as even as possible in relation to time and location and they help tokeep the difference between the base value of the surface temperatureand the temperature at the turnaround points as small as possible. Acombination of the measures can therefore keep this temperaturedifferential particularly small.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the process according to the invention are shown in thedrawings and are described below in more detail. In particular, thereare shown schematically, in

FIG. 1, a motion cycle with a constant translational velocity of theburner block and a constant peripheral velocity of the preform, by wayof velocity profiles, in

FIG. 2, a motion cycle with an increased peripheral velocity of thepreform in the turnaround point area, by way of velocity profiles, in

FIG. 3, a motion cycle where the flame temperature is being varied in atransitional area, by means of a temperature profile, in

FIG. 4, a motion cycle where the flame temperature is being varied in atransitional area, by means of a further temperature profile, in

FIG. 5, a motion cycle where in a transitional area the distance betweenthe burner orifice and the preform surface is being changed, by way of adistance profile, and in

FIG. 6, a device for the implementation of the process according to theinvention, in a side view.

DETAILED DESCRIPTION

A device for the implementation of the process according to theinvention is shown schematically in FIG. 6 as it was used in theexamples described in more detail below. A porous preform 62 isdeposited from SiO₂ particles on an aluminum oxide carrier 61 which isrotating about its longitudinal axis 63. The deposition of the SiO₂particles takes place by means of flame hydrolysis burners 65 of quartzglass, which are arranged in a row 66 on a burner block 64 which extendsparallel to the longitudinal axis 63 of the carrier 62. The burner block64 moves forward and back along the longitudinal axis 63 of the carrier61, between two turnaround points which are fixed in relation to thelongitudinal axis 63. The amplitude of the forward and back motion ischaracterized by the directional arrow 67. It measures 15 cm andcorresponds to the axial distance between the turnaround points orbetween the burners 65, as seen in the direction of the longitudinalaxis 63.

The row 66 of the burners 65 for the deposition is closed up on eachside by additional burners 68. The additional burners 68 are alsoinstalled on the burner block 64 and their distance from the nextclosest hydrolysis burner 65 is in each instance equal to the aboveburner distance. The flame temperature of the additional burners 68 isset to approximately the same value as those of the hydrolysis burners65. The additional burners 68 provide a similar temperature profile inthe end zones of the burner row 66 as exists in its central zone. Thehydrolysis burners 65 are each supplied with oxygen and hydrogen as fuelgases and with SiCl₄ as the starting material for the formation of SiO₂particles. The two additional burners 68 are supplied with fuel gasesonly.

Additional heating burners 69 are provided at the frontal sides of thepreform 62 and are fixedly installed in relation to the preform 62. Theheating burners 69 generate a higher temperature in the margin areas ofthe preform as compared to the hydrolysis burners 65 or the additionalburners 68. This results in increased density of both ends of thepreform 62 and thus in higher mechanical stability.

The temperature of the preform surface 70 is being monitored constantly.A pyrometer is aimed at the preform surface 70 for this purpose suchthat its measuring target is located in the impingement point of flame72 of one of the central hydrolysis burners 65a. The pyrometer 71 isfixed to the burner block 64 and moves with it forward and back. Thepyrometer is an "Infratherm IN 4/5" model made by IMPAC and has ameasurement wavelength of 5.14 μm. The measurement area is approximately5 mm and the distance of the pyrometer from the preform surface is 30cm. The burner flame impingement point has a diameter of about 15 mm.The measurements derived in this manner are used to determine the basevalue of the surface temperature as the lowest temperature in a motioncycle, i.e., a forward and back movement of the burner block 64.

The pyrometer 71 is connected with a control device 73 which regulatesthe fuel gas supply to the hydrolysis burners 65.

The distance between the surface 70 of the preform 62 and the burnerblock 64 is held constant during the deposition process. For this, theburner block 64 is movable perpendicular to the longitudinal axis 63 ofthe carrier 61, as indicated by directional arrow 74.

Diagrams in FIGS. 1 to 4 show the profiles of parameters as they arebeing changed during a motion cycle between two adjoining turnaroundpoints A and B. Each y-axis indicates the distance of the burner blockmotion between the turnaround points A and B, while the variousparameters of the SiO₂ deposition are recorded on the x-axis.

The parameter profile curves are provided with directional arrows whichindicate the applicable direction of the burner block motion for eachcurve section. For a clearer depiction the curves in the diagrams aredrawn next to each other (and not over one another) even when theparameter values are the same.

EXAMPLE 1

On the y-axis in FIG. 1, the peripheral velocity of the preform isplotted as v₁ and the translational velocity of the burner block is v₂.

The peripheral velocity v₁ is set at a constant 12 m/min during theentire motion cycle and during the entire deposition process. In thediagram, the portion of the curve showing the forward motion of theburner block toward the turnaround point B is represented by la, and thereverse motion of the burner block from the turnaround point B to theturnaround point A is marked lb.

The average translational velocity of the burner block is 500 mm/min(curves 2a and 2b). Apart from the slowing or accelerating distances 3in the area of the turnaround points A and B which are insignificant asfar as concerns the average translational velocity, v₂ is also keptconstant, both during the entire motion cycle and during the entiredeposition process (in terms of the rate). The lengths of the slowing oraccelerating distances 3 are in the range of a few millimeters.

The velocity profile shown in FIG. 1 is maintained during the entiredeposition process. During the deposition a surface temperature basisvalue of about 1,250° C., is measured on the preform surface. It shouldbe noted that as the outer diameter of the preform increases, thesurface cools off faster due to increased heat radiation. In order tomaintain the basis value of the surface temperature at a constant 1,250°C, measures are necessary to counteract the faster cooling off. For thispurpose the flame temperature of the hydrolysis burners is beingcontinually increased in this example.

The preform peripheral velocity v₁ as well as the burner block averagetranslational velocity v₂ are relatively small. Therefore the rate ofthe relative motion between the hydrolysis burners and the burner blockis also small and a good heating of the preform in the burner flameimpingement area is achieved.

The temperature increase due to the doubled heating of the preformsurface by the forward and back motion in the turnaround point areas Aand B therefore only amounts to 50° C. This assures a relatively flatsurface temperature gradient between the turnaround points A, B and thusa small density gradient of the preform in this area.

EXAMPLE 2

In FIG. 2. as well, the peripheral velocity of the preform is plotted onthe y-axis as v₁ and the translational velocity of the burner block asv₂.

The average translational velocity of the burner block is 800 mm/min(curve 4a and 4b). Apart from the braking or accelerating distances 5 inthe area of the turnaround points A and B which are insignificant as faras concerns the average translational velocity, the translationalvelocity is kept constant, both during the entire motion cycle andduring the entire deposition process (in terms of the rate). The lengthsof the braking or accelerating distances 5 are in the range of a fewmillimeters.

The preform peripheral velocity v₁ is controlled by a fixed program. Itis adjusted over a distance of about 9 cm between the turnaround pointsA and B at 12 m/min (curve sections 5a and 5b). During a forward motionof the burner block, for example toward the turnaround point B and fromabout 3 cm before it, the peripheral velocity v₁ is gradually increasedwithin about a 5 mm long transitional area 6c to 18 m/min (curve section6a). v₁ is then kept at this higher value until the turnaround point B(curve section 7a). During the reverse motion of the burner block fromthe turnaround point B, this same velocity profile is reversed (curvesections 7b, 6b, 5b). The same velocity profile is run correspondinglyduring the forward motion of the burner block toward the turnaroundpoint A. The average peripheral velocity in the motion cycle is about 14m/min. This velocity is schematically indicated in FIG. 2 by a dottedline 8.

The velocity profile represented in FIG. 2 is maintained during theentire deposition process. The measured base value of the preformsurface temperature is 1,280° C. during the deposition process. As faras the maintenance of this surface temperature is concerned while theouter diameter of the preform increases, the information provided abovein respect to FIG. 1 applies here. A constant surface temperature isnecessary if a radial density gradient in the preform is to be avoided.

The average peripheral velocity of the preform and the averagetranslational velocity of the burner block are relatively small.Therefore the rate of the relative motion between the hydrolysis burnersand the burner block is also small and a good heating of the preform inthe burner flame impingement area is achieved. In addition, in the areaof the turnaround points A and B, the preform receives less heating persurface unit due to the higher peripheral velocity so that thetemperature increase caused by the doubled heating of the preformsurface during the forward and back motion in the turnaround point areasA and B can be kept very small. It only amounts to 40° C.

EXAMPLE 3

The preform peripheral velocity v₁ and the burner block translationalvelocity v₂ are regulated according to the example explained by FIG. 1.A constant surface temperature base value of 1,250° C. is maintained atthe preform surface during the deposition process. In addition, in thisexample the flame temperature of the hydrolysis burners is being variedin the turnaround point area of the burner block motion.

The variation of the flame temperature is controlled by a program and isexplained by means of FIG. 3. The flame temperature "T" of thehydrolysis burners is plotted in relative units on the y-axis.

In a central area between the turnaround points A, B (curve sections 9a,9b) the flame temperature is kept at a high level. During the forwardmotion of the burner block, for example in the direction toward theturnaround point B and approximately 3 cm before it, the flametemperature is continually lowered in a transitional area 10c (curvesection 10a). The transitional area 10c ends at the turnaround point B.

In a first process variant the flame temperature is lowered by acontinual reduction of the fuel gas supply, by a total of 8% of theinitial amount as was set immediately preceding the transitional area10c. At the same time the ratio of oxygen and hydrogen is kept constant.The supply of the remaining gases to the hydrolysis burners is also leftunchanged.

In a second process variant the flame temperature is lowered bysupplying nitrogen to the burners. For this the flow of nitrogen iscontinually increased in the traditional area 10c until it reaches about20% of the hydrogen supply.

In a third process variant the flame temperature is lowered byadditional supply of SiCl₄ to the burners while the fuel gas supply iskept constant. For this the flow of SiCl₄ is being continually increasedup to about 20% of the initial amount as it was set immediatelypreceding the transitional area 10c.

When the burner block is traveling back from the turnaround point B theflame temperature is again gradually increased in a further transitionalarea 10d until it reached the original value (curve section 10b);however, the temperature increase is somewhat slower than thetemperature decrease in the curve section 10a. The reason for this isthat during the reverse travel the surface temperature of the preform isstill raised in the area adjoining the turnaround point due to theheating that took place in the course of the forward motion. In order toavoid an additional heating of this area by a rapid flame temperatureincrease, the temperature is increased slower to its original value thanin the transitional area 10c, until it reaches the original temperaturein curve section 9b. High temperature differences and thus densityvariations are avoided by means of these transitional areas 10c, 10dwith their varying lengths.

The same purpose is achieved by a process variant where the flametemperate during the reverse travel from the turnaround point B is atfirst being kept constantly low during a certain distance and isincreased only later, as will be explained by way of a similar processin example 5.

In all process variants the increase of the flame temperature takesplace by restoring the original gas supply rates to the hydrolysisburners.

The preform is subject to lower heat output in the area of theturnaround points A, B due to the lowering of the flame temperature, sothat the temperature increase caused by the double heating of thepreform surface as a result of the forward and back motion in the areaof the turnaround points A, B only amounts to 35° C.

In a further process variant the flame temperature of the hydrolysisburners can be set by means of a control device. The reference surfacetemperature for the purposes of control is set at 1,250° C. The preformsurface temperature is continually measured at the flame impingementpoint by means of a pyrometer. In case of a temperature change, forexample in case of a temperature increase in the turnaround point area,the flame temperature of all hydrolysis burners is adjusted by thecontrol device through a change of one or more gas flows supplied to thehydrolysis burners. The above indicated process variants are suitablefor a change of the flame temperature. The control device in particularcontributes to an avoidance of an unacceptable temperature increase atthe turnaround points and makes possible the maintenance of a constantpreform density over the entire duration of the deposition. In this waythe temperature increase in the turnaround point areas can be limited toless than 30° C.

EXAMPLE 4

The deposition parameters illustrated by the example in FIG. 4 are setaccording to the process variant described by way of FIG. 3. However, incontrast to the temperature profile "T" represented in FIG. 3, the flametemperature is being constantly varied by programmed control during themotion cycle. Therefore the areas of constant flame temperature (curvesections 9a and 9b in FIG. 3) are absent in this temperature profile.

During the forward motion of the burner block toward the turnaroundpoint B the burner flame reaches its maximal temperature at a point 11a,approximately 6 cm before the turnaround point B. Afterward the flametemperature is continually reduced in a transitional area 12c (curvesection 12a) and reaches its minimum temperature at the turnaround pointB. Thus the transitional area 12c ends there. The lowering of the flametemperature takes place according to the process variants describedabove by way of FIG. 3.

During the reverse travel of the burner block the flame temperature isagain gradually returned to its maximal temperature in a furthertransitional area 12d, reaching the maximum at point 11b of thetemperature profile (curve section 12b). The transitional area 12d endsapproximately 6 cm before the turnaround point A; it extends thus about9 cm from the turnaround point B. An overheating of the preform in itsregion adjoining turnaround point B is thus prevented by the slowertemperature increase in the transitional area 12d during the reversetravel as was already explained in more detail be way of FIG. 3. In thisexample, the transitional areas 12d, 13d overlap during the reversetravel from the turnaround points A and/or B.

In this process variant, irregularities of the flame temperature areavoided. In addition to the measures already explained by way of FIG. 1,the flame temperature is lowered in the area of the turnaround points A,B. In this way the preform is subjected to a lesser heat output, so thatthe temperature increase due to the double heating of the preformsurface caused by the forward and back motion in the area of theturnaround points A, B merely amounts to 35° C.

EXAMPLE 5

In this example the peripheral velocity of the preform and thetranslational velocity of the burner block are set according to theexample explained by way of FIG. 1. During the deposition process aconstant surface temperature base value of 1,250° C. is measured at thepreform surface.

In this example the low temperature increase in the area of theturnaround points is the result of a change in the distance of theburner orifices from the preform surface. This distance "D" is plottedon the y-axis of the motion cycle represented in FIG. 5. When the motioncycle is viewed during the movement of the burner block in the directiontoward the turnaround point B, it is apparent that between theturnaround points A, B the distance is kept steadily small over aninterval 14c from about 9 cm to about 15 cm (curve section 14a), and issubsequently gradually increased in a transitional area 15b at aconstant rate of change of 7.5 mm/s (curve section 15a). Thetransitional area 15b ends at the turnaround point B; there the distancebetween the hydrolysis burner orifice and the preform is about 10%greater than in the curve section 14a; it amounts therefore to about16.5 cm.

During the reverse travel of the burner block the distance is kept at ahigher value over an interval 16b of about 2.5 cm (curve section 16a)and is subsequently gradually reduced to the original distance of 15 cmat a rate of change of 7.5 mm/s in a transitional area 17b (curvesection 17a). In the course of further travel of the burner block towardthe turnaround point A this distance is again kept constant over aninterval of about 9 cm (curve section 14b) and the same distance profileis subsequently executed as was explained above in reference toturnaround point B.

The delayed adjustment to the smaller distance during the reverse travelfrom the turnaround points A, B (curve section 16a) causes a slowerheating of the preform surface which is still heated up as a result ofthe forward motion and thus prevents an overheating of the preformsurface in the areas around the turnaround points A, B.

Increase of the distance between the hydrolysis burners and the preformsurface in the turnaround point area additionally contributes to as evena heating as possible of the preform over its length. A temperatureincrease of merely 35° C. was measured in the area of the turnaroundpoints A, B.

We claim:
 1. A process for the production of quartz glass bodies bydeposition of SiO₂ particles on the mantle surface of a cylindricalcarrier rotating about its longitudinal axis and forming an elongatedporous preform, said process comprising:forming the SiO₂ particles in aplurality of hydrolysis burners which are arranged in at least oneburner row disposed parallel to the longitudinal axis of the carrier;providing for relative reciprocating movement of the burner row relativeto the preform parallel to the longitudinal axis of the carrier so thateach burner reciprocates between respective turnaround points locatedbetween longitudinal ends of the preform at which turnaround points thedirection of reciprocating movement is reversed, where the burners moveat preset transitional speed forward and back between respectivetransitional areas adjacent the turnaround points in which transitionalareas the burners move slower that said preset transitional speed; andsintering of the preform thus produced; wherein, when the burners are inthe transitional area adjacent the turnaround points, the peripheralvelocity of the forming preform is increased, the flame temperature ofthe hydrolysis burners is lowered, or the distance of the hydrolysisburners from the preform surface is changed.
 2. A process according toclaim 1, wherein the turnaround points are kept at a distance in a rangebetween 5 cm and 40 cm.
 3. A process according to claim 1 and furthercomprising a warming burner positioned bewteen two adjoining hydrolysisburners in the burner row.
 4. The process according to claim 1, whereinthe hydrolysis burners are supplied with fuel gases and an inert gas ata controllable rate, the flame temperature being lowered by increasingthe rate of inert gas supply.
 5. A process according to claim 1 whereinthe hydrolysis burners are supplied with fuel gases and other gasesflowing at controllable rates, and wherein the flame temperature islowered by reducing the rate of flow of the fuel gases in relation tothe rate of flow of the other gases being supplied to the hydrolysisburners.
 6. A process according to claim 1 wherein the flame temperatureis lowered by supplying inert gas to the hydrolysis burners.
 7. Aprocess according to claim 1 wherein the hydrolysis burners are suppliedwith starting material at a supply rate, and oxygen and/or hydrogen, theflame temperature being reduced by increasing the rate of supply of thestarting material to the hydrolysis burners for the formation of SiO₂particles relative to the supply of oxygen and/or hydrogen.
 8. A processaccording to claim 1 wherein the increase of the peripheral velocity ofthe preform, the decrease of the flame temperature of the hydrolysisburners or the change of the distance of the hydrolysis burners from thepreform surface within the transitional areas ending before or at theturnaround points takes place gradually.
 9. A process according to claim8, wherein each of the transitional areas during the forward motion ofthe burner row toward the turnaround points is shorter than thetransitional area during the reverse motion away from the turnaroundpoints.
 10. A process according to claim 1 wherein the preform surfacetemperature is measured in a flame impingement area of one of thehydrolysis burners and the resulting measurement value is used forregulating the rotational velocity of the carrier, the flame temperatureof the hydrolysis burners and/or the distance of the hydrolysis burnersfrom the preform surface.
 11. The process according to claim 1 whereinthe relative reciprocating movement is provided by the burner nowremaining substantially stationary and the preform moving reciprocallyalong a longitudinal path.
 12. The process according to claim 1 whereinthe relative reciprocating movement is provided by the preform remainingsubstantially stationary and the burner row moving reciprocally along alongitudinal path.
 13. A process for the production of quartz glassbodies by deposition of SiO₂ particles on the mantle surface of acylindrical carrier rotating about its longitudinal axis and forming anelongated porous preform, said process comprising;forming the SiO₂particles in a plurality of hydrolysis burners which are arranged in atleast one burner row disposed parallel to the longitudinal axis of thecarrier; providing for relative reciprocating movement of the burner rowrelative to the preform parallel to the longitudinal axis of the carrierso that each burner reciprocates between respective turnaround pointslocated between longitudinal ends of the preform at which turnaroundpoints the direction of reciprocating movement is reversed, where theburners move at a preset transitional speed forward and back betweenrespective transitional areas adjacent the turnaround points in whichtransitional areas the burners move slower than said preset transitionalspeed; and wherein the distance between turnaround points issubstantially equal to the distance between burners in the burner row;and sintering of the preform thus produced; wherein, when the burnersare in the transitional areas adjacent the turnaround points, theperipheral velocity of the forming preform is increased, the flametemperature of the hydrolysis burners is lowered, or the distance of thehydrolysis burners from the preform surface is changed.